A hollow molded product of a synthetic resin is made by injecting a molten resin into a mold cavity, and forcing a fluid, such as nitrogen gas or water, into the resin to form a hollow interior.
A known mold for hollow injection molding used for making such a hollow molded product is described in U.S. patent to Hendry, U.S. Pat. No. 4,474,717, Oct. 2, 1984, and shown in FIG. 8 herein.
Referring now to FIG. 8, the known mold has a cavity 91 for making a hollow molded product, a sprue 92 for injecting a molten synthetic resin 920 into the cavity 91, and a fluid inlet 931 for admitting a fluid 930 for forming a hollow interior in the molded product. The fluid inlet 931 is defined by the open end of a fluid admitting member 93 which stays in a protruded form in the cavity 91.
The fluid admitting member protruding into the cavity in the known mold, however, allows the synthetic resin 920 to reversely flow into the fluid inlet 931 defined by the fluid admitting member when the resin is injected, or when the fluid is discharged. As a result, the fluid inlet 931 is closed by the synthetic resin 920. Therefore, it has been very difficult to discharge the fluid.
It is possible to prevent the synthetic resin 920 from flowing into the fluid inlet 931 if the cross section of the fluid inlet is very small. This, however, in turn makes it impossible to admit a sufficiently large amount of fluid within a given time.
Providing the fluid admitting member 93 with the fluid inlet 931 necessarily increases the dimensions of the fluid admitting member. A fluid admitting member of such dimensions protruding into the injected resin will deprive the resin of a large amount of heat. The resin which has been deprived of a large amount of heat forms a hardened thick layer contacting the fluid admitting member. This layer prevents fluid from flowing into the synthetic resin through the fluid inlet.